Investigations of gene regulation in B lymphocytes continue to focus on two model systems: the human kappa light chain immunoglobulin gene and the human J chain gene. The expression of the kappa immunoglobulin gene si regulated by a transcriptional enhancer located in the J-C intron. We have studied two domains within the enhancer that bind nuclear proteins. A protein which binds to the 3' domain (at the kappaE2 site) has been further purified (now approximately 5000-fold) and a molecular weight estimate has been obtained by UF-crosslinking/SDS-PAGE analysis. In investigations of proteins binding to the 5' domain of the enhancer, we detected two forms of binding activity at the kappaB motif by an exonnuclease protection assay of nuclear extracts; we identify these as NFkappaB1 and NFkappaB2. The latter activity is widely distributed, even in cells that do not transcribe kappa. NFkappaB1 activity, however, seems to be confined to cells in which the kappa gene is transcribed; it may be the functional form that mediates enhancer activity. In nuclear extracts from several sources in which the enhancer is inactive, NFkappaB1 binding activity could be induced by in vitro treatment with guanidine at millimoler concentrations. It is possible that the induction observed in vitro with guanidine may occur by a mechanism similar to the in vivo induction of NFkappaB reported in response to lymphokines. The J chain promoter region has been analyzed by transfecting into a myeloma a series of constructs in which the promoter--with various deletions--is linked to a chloramphenicol acetyl transferase (CAT) gene. Deletions in three regions of the promoter have been found to affect CAT transcription in a positive or negative way. In vitro investigations of nuclear extract proteins capable of binding in the promoter region have detected several bonding sites, at least one of which appears to coincide with functionally important sequence (as demonstrated by the deletion analysis).